Flicker and three flashes, then the sequence stops.

Three flashes only: U10 PIA

Cause: The U10 6821 PIA or its socket, a corroded trace to U10, or U20 (4502) which feeds it.

Checks

• Inspect U10 traces near the battery.
• Try a known good 6821 (U10 and U11 are interchangeable).

Fix

• Replace U10, its socket, or U20; repair broken traces.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35
• Stern MPU-100
• Stern MPU-200

Related tips

• Four flashes only: U11 PIA, R134 or a bad display
• One PIA output does not toggle in the test EPROM test
• Keep spare 6800, 6821 and 5101 chips
• CPU flickers then gives three flashes: bad PIA at U10
• One flash only: U7 6810 RAM
• Two flashes only: U8 5101 RAM
• Five flashes only: U12 timer or open R113
• Leon code running but LEDs dark: bad CPU PIA IC18

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Two, Clay Harrell (pinrepair.com)